The present invention relates to a chuck assembly which is mainly mounted on a spindle shaft of an electric drill, a driver drill or a vibration drill, and more particularly to a chuck assembly used in a screw fastening operation or a boring operation with a tool such as a drill or bit.
A chuck assembly as shown in FIGS. 1A and 1B are conventionally known in which three jaws 2 each of which has an axis L.sub.2 slanted relative to an axis L.sub.1 of a chuck body 1 are provided on the chuck body 1, a male screw portion 2a provided on each jaw 2 is engaged with a female screw portion 4a of a nut member 4 rotated by the rotation of a rotary member 3 provided on the chuck body 1, and the nut member 4 is rotated by the rotation of he rotary member 3 to open/close and advance/retract the jaws 2 to grip a shank 5a of a tool 5. Reference numeral 9 denotes a spindle of an electric drill.
An object of such a chuck assembly is to perform the work by rotating the nut member 4 to push a tool grip portion of each jaw 2 against the outer circumferential surface of the shank 5a of the tool 5 to grip a tool 5 and rotating it. Accordingly, it is necessary to keep an axis vibration precision and a grip rigidity over the full range of the grip diameter.
By the way, in the case where the tool grip portion of each jaw 2 is kept under the condition that it is at one edge E or adjacent to an edge as shown in FIGS. 1A and 1B, the nut member 4 is rotated so that the tool grip portion of the jaw 2 is lightly contacted with the outer circumferential surface of the shank 5a of the tool 5 under a neutral condition with respect to the backlash as shown in FIGS. 2A to 2F. Thereafter, the nut member 4 is strongly rotated in a fastening direction and the tool grip portion of the jaw 2 is depressed against the shank 5a. As a result, the tool grip portion of the jaw 2 is rotated by the rotation of the nut member 4 so that the tool grip portion firmly holds the shank 5a as shown in FIGS. 3A to 3F. (In FIG. 3E, the arrow indicates the rotational direction of the nut member 4 when the shank 5a is fastened, and the reference character S indicates the locus of the tool grip portion.) When the electric drill or the like is rotated under this condition, a rotational torque is transmitted from an axial hole of the jaw 2 through the chuck body 1 to the jaw 2 so that the tool grip portion of the jaw 2 is rotated as shown in FIGS. 4A to 4F (In FIG. 4E, the arrow indicates the direction in which the rotational torque is applied) and the rotational torque is transmitted from the tool grip portion of the jaw 2 to the tool 5.
Thus, the force for rotating the jaws 2 is applied in the opposite directions between the case where the rotational torque is applied from the chuck body 1 upon machining and the case where the nut member 4 is rotated to grip the tool 5 by the tool grip portions of the jaws 2 (before machining).
Namely, in the case where the shape of the tool grip portion of each jaw 2 is at the edge E or adjacent to the edge as shown in FIGS. 1A to 1F, due to the phenomenon that the above-described jaw 2 is rotated about the axis L.sub.2, the force is applied to the tool grip portion of the jaw 2 so that the grip portion on the front side from a point P in FIGS. 2F, 3F and 4F bites into the outer circumferential surface of the shank and the grip portion on the rear side from the point P is shifted in a direction away from the outer circumferential surface of the shank. In the case where the shank 5a of the tool 5 is thus gripped along the single line E, it is difficult to prevent the rotational phenomenon about the axis L.sub.2 of the jaw 2 so that the axis vibration precision becomes worse.
In general, the jaws 2 and the shank 5a are made of steel material. The steel is not rigid but elastic material. Furthermore, the hardness of the tool grip portion of each jaw 2 is generally set at a higher level than the hardness of the shank 5a of the tool 5. Accordingly, when the nut member 4 is rotated so that the grip force by the jaws 2 is increased, the tool grip portions of the jaws 2 are brought into contact with the shank 5a of the tool 5 due to the bite phenomenon of the tool grip portions or the distortion of the grip portions. The line E of the tool grip portion of each jaw 2 relative to the axis L.sub.3 of the tool 5 is slanted by the rotational phenomenon in contact with the outer circumferential surface of the shank 5a. In case of the large shank diameter, the curvature of the shank diameter is large. Accordingly, the grip condition of the grip portion on the rear side from the point P is not so bad. However, in case of the smaller shank diameter, in spite of the fact that the curvature of the shank diameter is small, the amount of the rotational phenomenon of the tool grip portion of each jaw 2 is the same as that in case of the larger diameter shank diameter (if the same person fastens the nut member 4 with the like force). The contact of the grip portion with the smaller shank outer circumferential surface becomes small. Accordingly, in comparison with the larger diameter shank, the stability of the grip portion of the smaller diameter shank becomes much worse. Also, the axis vibration precision would not be stabilized.
As described above, in the case where the shank surface of the tool is gripped by the single line E, there is a small effect to prevent the rotational phenomenon about the axis L.sub.2 of the jaw 2, resulting in the poor stability in the axis vibration precision.
Accordingly, there has been another approach for maintaining the axis vibration precision at a satisfactory level, in which three jaws 2 are kept under the open condition, and the tool grip portions of the three jaws 2 are machined with a grinding/cutting tool while rotating the jaws 2. However, this process is very troublesome.
Furthermore, in the case where the tool grip portions of the jaws 2 are machined in the above-described process, in the case where the shank diameter for gripping is greater than the inner diameter obtained by machining (see FIG. 5), the grip is effected by two edges at points B and H. The shank is gripped by the two edge lines over the full length of the grip portion. Even if the nut member 4 is rotated to close and advance the jaws 2 or even if the rotational torque is transmitted from the chuck body 1, the outer circumferential surface of the shank is gripped by the two edge lines. As a result, the phenomenon in which the jaws are rotated about the axis L.sub.2 is prevented by the respective edge lines. Also, the axis vibration precision would be stabilized. Also, although the edges are at obtuse angles, the grip rigidity may be kept to some extent by the slight bite phenomenon of the edge lines to the shank. (In general, the hardness of the tool grip portion of jaws 2 is set at a somewhat higher level than that of the shank 5a of the tool 5.)
However, in the case where the shank diameter for gripping is smaller than the inner diameter obtained by machining (see FIG. 6), the grip is effected by the point E on the arcuate line of the tool grip portion of the jaw 2, and the grip portion is in contact with the shank of the tool 5 at the single line E over the full length of he grip portion. As a result, it is difficult to prevent the rotational phenomenon of the jaw 2 about the axis L.sub.2 as described above. The posture of the jaw 2 is not stabilized. Also, the axis vibration precision would become unstable. (In general, in case of the smaller diameter tool, it is unnecessary to keep a high grip rigidity but a high speed rotation is needed so that the vibration precision is particularly needed.)
Also, there is another means for enhancing the precision of each part of the chuck assembly as much as possible as a maintenance means for the axis vibration precision. The current situation thereof is such that the quality control of each part is very difficult.
By the way, as disclosed in EP 0530 431 B1 and as shown in FIGS. 7 and 8, there is another approach in which a shape of the tool grip portion of the jaw 2 takes a pair of asymmetrical edges relative to the axis L.sub.2 of the jaw 2 to expect the bite of the shank 5a of the tool 5.
However, in this case, either one of the two edges is advanced to come into contact with the shank outer circumferential surface depending upon the shank diameter as shown in FIGS. 7 and 8 due to the magnitude of the shank diameter of the tool 5. The phenomenon in which the jaw 2 is rotated about the axis is generated upon fastening the tool 5 by rotating the nut member 4, and the rotational phenomenon is generated in the opposite direction during the machining work. Accordingly, even if the maintenance of the grip rigidity is possible according to this technology, it is impossible to obtain a satisfactory axis vibration precision.